always use engine (#1525)

<!-- CURSOR_SUMMARY -->
> [!NOTE]
> Always route Dyad Pro through the Dyad engine (no gateway), simplify
logging/flags, and remove DYAD_GATEWAY usage and staging gateway script.
> 
> - **Engine/Model client (`src/ipc/utils/get_model_client.ts`)**:
> - Always use `createDyadEngine` when Dyad Pro is enabled; remove
gateway fallback and `DYAD_GATEWAY_URL` usage.
> - Simplify logs to engine-only; drop conditional gateway/engine
logging.
> - Always set `isEngineEnabled: true`; pass `{ files }` to the provider
unconditionally; strip `:free` from model names.
> - **Scripts (`package.json`)**:
>   - Remove `staging:gateway` script.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
5fdbbb703c47de4623ceeb82e6678ace34dc268e. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

package.json

@@ -16,7 +16,6 @@
     "start": "electron-forge start",
     "dev:engine": "cross-env DYAD_ENGINE_URL=http://localhost:8080/v1 npm start",
     "staging:engine": "cross-env DYAD_ENGINE_URL=https://staging---dyad-llm-engine-kq7pivehnq-uc.a.run.app/v1 npm start",
-    "staging:gateway": "cross-env DYAD_GATEWAY_URL=https://staging---litellm-gcp-cloud-run-kq7pivehnq-uc.a.run.app/v1 npm start",
     "package": "npm run clean && electron-forge package",
     "make": "npm run clean && electron-forge make",
     "publish": "npm run clean && electron-forge publish",